化学家教 Chemistry Tutor

化学家教多年经验专业辅导各个级别的化学课程，包括十年级科学，十一年级和十二年级化学，AP化学， IB化学，SAT化学UBC和LANGARA大学一年级普通化学和大学二年级有机化学，物理化学和分析化学。张老师从南开大学化学系博士毕业，在UBC化学系工作8年，具有牢固的化学知识基础和丰富的实际工作经验，如果您有家教需要，请和张老师联系，电话：604-808-6827， 电邮：vancouverchemistrytutor@gmail.com

AP Chemistry Question of the Day --61

Welcome back to my blog. here is Question 60 I posted earlier.
60. Glycerol has lower vapour pressure than does ethanol at room temperature. Which of glycerol and ethanol has the lower boiling point?

The correct answer should be ethanol.

Vapour pressure is the pressure of the vapour when at a certain temperature the liquid and the vapour reach equilibrium in a closed container. Vapour pressure is only dependent on the nature of the substance and temperature. For a given substance, its vapour pressure is only dependent on temperature.  As temperature increases, vapour pressure increases. When the vapour pressure reaches the atmosphere pressure, the temperature is defined as boiling point. As a result, the higher the vapour pressure, the lower the boiling point.

Following is question 61 of the series:

 61. In an experiment to determine the percentage of water in a solid hydrate by heating, what is the best indication that all the water has been removed?
(A)  The solid melts.
(B)  The solid changes color.
(C)  Water vapor no longer appears.
(D)  Successive weighings give the same mass.

I will post the answer in my next blog.
Thank you very much for visiting my blog. I am available to discuss any topics in Chemistry at all levels. Please shoot me an email at vancouverchemistrytutor@gmail.com or give a call at 604-808-6827.

2014 AP Chemistry Score released

The 2014 AP Chemistry score has been released. Overall it is harder to get a 5 than in 2013.
Here is the stat on the average 2014 scores:

AP Score	Total Points Required	Percentage of AP Chemistry Examinees Who Achieved These Points
5	72-100	10.1%
4	58-71	16.6%
3	42-57	25.7%
2	27-41	25.8%
1	0-26	21.8%

Here is a comparison between the score distribution between 2013 and 2014.

AP Score	2013	2014
5	18.9%	10.1%
4	21.5%	16.6%
3	18.8%	25.7%
2	14.9%	25.8%
1	26.0%	21.8%

In general, it is harder to get a high score this year. 

The major contributor to this discrepancy is the introduction of the new curriculum framework. The free response part of the exam was overwhelming and most of the students (73%) did not finish.

If you want to learn AP chemistry with me, give me a call at 604-808-6827 and I am more than happy to help you in achieving a 5 in 2015.

AP Chemistry Question of the Day --60

Welcome back to my blog. Here is question 59 I posted last week.

59.  Which of the following best helps to account for the fact that the F^- ion is smaller than the O^2- ion?                                                     

(a) F^- has a larger nuclear mass than O^2-    

(b) F^- has a larger nuclear charge than O^2-   

(c) F^- is more electronegative than O^2-        

(d) F^- is more polarizable than O^2-.

The correct answer is b.  Both fluoride and oxygen ions have 10 electrons. They are isoelectronic to each other. The size of the ions is determined by the number of protons in the nucleus. Since fluoride has 9 and oxygen has 8 protons. The electrons in fluoride ion experience more attraction from the nucleus and makes the ions smaller.

Statement A and C are both correct but they are not the right explanation.

Here is question 60 of the series:

60. Glycerol has lower vapour pressure than does ethanol at room temperature. Which of glycerol and ethanol has the lower boiling point?

Thank you very much for visiting my blog. I am available to discuss any topics in Chemistry at all levels. Please shoot me an email at vancouverchemistrytutor@gmail.com or give a call at 604-808-6827.